Optimization of the properties of bulk heterojunctions obtained by coevaporation of Zn-phthalocyanine/perylene

Abstract

Zn-phthalocyanine (ZnPc)–perylene tetracarboxylic dianhydride (PTCDA) “bulk heterojunctions” has been obtained by vacuum evaporation from a single heated source. The composition, in weight, of the initial powder was 0, 10, 25, 50, 100% of PTCDA. The deposited films have been characterized by infrared, visible and near UV absorption, scanning electron microscopy and X-ray photoelectron spectroscopy (XPS). “Diodes bulk heterojunctions” ITO/ZnPc:PTCDA/Al and ITO/PEDOT:PSS/ZnPc:PTCDA/Al have been also realized and studied by I – V characteristic measurements in the dark and under illumination (ITO: indium tin oxide; PEDOT:PSS: poly(3,4-ethylene dioxythiophene) oxidized (doped) with poly(4-styrenesulfonate)). It is shown, in the case of pure ZnPc or PTCDA thin films that the properties of the initial molecules are preserved, during the deposition process. The introduction of PTCDA in the ZnPc induces a best covering of the visible spectrum. It is shown that the films with the optimum PTCDA concentration in ZnPc (25% weight) for the expansion of light absorption spectrum give the best results in solar cells. The formation of a blend PTCDA:ZnPc was found to improve the photovoltaic performances of the solar cell. This improvement is attributed to the expansion of light absorption spectrum and to charge separation by the bulk heterojunctions. Moreover, the presence of a thin PEDOT:PSS film at the interface ITO/ZnPC:PTCDA allows one to achieve a significant solar cell efficiency of nearly 0.2%.